Secondary batteries having electrical characteristics such as easy applicability to products and high energy density are broadly used not only in portable devices but also in electric vehicles (EVs) or hybrid vehicles (HVs) which are driven using electric power sources.
The secondary batteries may greatly reduce use of fossil fuel, do not produce any by-product after using energy, and thus are regarded as a new environment-friendly energy source capable of increasing energy efficiency.
A battery pack used in an electric vehicle or the like typically includes a plurality of battery cells connected to each other in series and/or in parallel. The battery cells include a positive current collector, a negative current collector, a separator, an active material, an electrolyte, etc., and are rechargeable due to electrochemical reaction among the components thereof.
The battery pack generally includes a battery management system (BMS) configured to control power supply, measure electrical characteristic values such as a current and a voltage, control charge/discharge, control voltage equalization, estimate the state of charge (SOC), estimate the state of health (SOH), etc.
The battery cells included in the battery pack may have deviation in performance due to various reasons. The deviation in performance of the battery cells causes voltage imbalance among the battery cells.
When the battery pack having voltage imbalance is used, the performance of the battery pack depends on a degraded battery cell and thus is reduced overall. In addition, since degradation of the degraded battery cell is accelerated as time passes, when the degraded battery cell is neglected, the life of the battery pack is rapidly reduced.
As conventional technologies capable of solving the above problems, passive balancing has been performed by selectively connecting a resistor to two ends of a battery cell and discharging the battery cell, and active balancing has been performed by transmitting power of a battery cell having a high voltage via a capacitor to a battery cell having a low voltage.
However, the conventional passive balancing technology consumes power by using the resistor for balancing and thus wastes power, and the conventional active balancing technology using the capacitor should repeatedly perform switching operations to generate a voltage difference between the battery cell and the capacitor and thus requires much time for balancing.